The "spectrophone" is useful as apparatus for detecting traces of an impurity in a gas medium, such as nitric oxide (NO) pollutant in air. The spectrophone includes an absorption cell containing the gas medium, upon which is incident optical radiation, typically infrared, which is modulated at an acoustic frequency. For example, laser radiation from a carbon monoxide laser is modulated in intensity at 150 Hz by a mechanical chopper. The useful output of the absorption cell is in the form of pressure variations at the acoustic frequency, generated in the gas by the absorption of the modulated light by the impurity. These pressure variations are detected with a microphone. In such a system, however, it is very unlikely that the laser wavelength will ordinarily coincide with the optimum absorption wavelength ("absorption line") of the impurity; therefore, ordinarily a continuously tunable laser, such as a spin flap Raman laser, must be used as the optical radiation source, in order to enable an exact coincidence to be obtained. The use of such tunable lasers as optical sources in general is commercially undesirable from the standpoints of apparatus complexity and expense. Accordingly, it would be desirable to have spectrophone apparatus for air pollution detection without the need for tuning of the laser source.